The only organs that exhibit any sign of degeneration are those of sense, but in the ectoparasitic Trematodes simple eye-like structures are present and perhaps serve as organs of temperature.
In the ectoparasitic Trematodes this post-oral sucker is a complex disk placed near the hinder end and provided With suckerlets, hooks and a musculature arising from a special skeleton.
If ectoparasitic and attached to the skin, apertures or gills, the Trematode adopts more elaborate adhesive organs and undergoes a less complex development than are required for the endoparasitic members of the class.
It is usual to consider the ectoparasitic habit as leading up to the endoparasitic one.
It has been compared with the canal of Laurer of Trematodes (the vitello-intestinal duct of the ectoparasitic flukes), but if we take the more primitive Cestodes, and especially Aniphilina, into consideration we find that they possess, in addition to the uterus, an anterior vagina (usually present in Cestodes) and a posterior one.
